1. Field of the Invention
The invention relates in general to a liquid crystal display apparatus in which the display image is written into a liquid crystal cell by irridating the cell with a laser beam and in which the image is enlarged and projected on a screen by optical means. Particularly, the invention relates to an apparatus in which the writing time into the liquid crystal cell is reduced so as to improve the quality of the display.
2. Description of the Prior Art
Previously, liquid crystal display apparatuses have been known in which the characters or the display images are written into the liquid crystal cell by irridating the cell with the laser beam and the image is then enlarged and displayed on the screen by optical projection means consisting of a light source, lens and a filter.
In such liquid crystal display apparatuses, image writing in the liquid crystal cell is achieved using the following principle. The laser beam is irridated on the liquid crystal cell for converting the laser beam energy into thermal energy. The heat thus generated is used to heat the liquid crystal to cause a phase transition in the heated spot of the cell. As the irridation of the laser beam is terminated, the liquid crystal tends to revert to its starting phase. However, the liquid crystal spot that has undergone a phase transition cools rapidly so the disrupted state of the molecular disposition of the liquid crystal caused by the phase transition is maintained and the light scattering center may remain in the liquid crystal spot. This causes the image to be stored or written into the spot.
More specifically image writing can be achieved by turning the laser beam on and off in a controlled manner with simultaneous scanning for sequentially writing picture elements or pixels that are the minimum size unit of the image. The laser beam can be scanned by the so-called raster scanning and vector scanning. In raster scanning, the laser beam is scanned linearly from, for example, the left side end towards the right side end of the liquid crystal cell and when the beam reaches the right side end of the cell it is promptly returned to the left side end but at a position slightly below the starting point for the preceding scanning line. The beam again resumes linear scanning towards the right side end of the cell and this sequence is repeated vertically from the top toward the bottom of the cell. In vector scanning, the laser beam is scanned randomly.
It should be noted that when the image is written into the liquid crystal cell by the described raster scanning, that the state of the image writing in a given cell spot corresponding to one pixel depends substantially on whether the writing is also performed in the immediately preceding and succeeding cells on the same scanning line. That is, when writing is also accomplished in the two adjacent blocks of the particular block being considered, more string writing is obtained for the same amount of energy supplied to the intermediate spot, that is, the product of the laser output multiplied by the writing time necessary for writing in a spot corresponding to each pixel than in the case in which the writing is not performed in any of the immediately adjacent spots. This is because the heat received by the immediate adjacent spots will be transmitted to the intermediate spot in which writing is to be effected, thus, facilitating writing in the intermediate spot. It will be noted that the scanning speed of the laser beam depends on the writing time per spot which is usually set to a constant value. In other words, for an equivalent writing quality a larger energy is required in the case where writing did not occur in the immediately adjacent spots than for the case where writing occurs in one or both of the immediately adjacent spots.1
Thus, it has been conventional practice to set the scanning speed of the laser beam by taking as a reference the case of the greatest difficulty of writing which is the case in which writing is not accomplished in any of the immediately adjacent spots. However, this results in a prolonged writing time for each liquid cell. On the other hand, when the scanning speed of the laser beam is set by taking as a reference the case of the greatest ease of writing, that is, the case in which writing is performed in one or both of the immediately adjacent spots, the writing time per liquid crystal cell is produced. However, for this case, the optimum writing is not acheived in the spot in which the writing is to be effective, thus resulting in a lower display quality. For this case, an optimum writing is possibly achieved by increasing the laser output, however, since the laser unit usually employed for writing is a semiconductor laser which is used at a constant laser output of 30 mW which is the presently available maximum power, it has not been feasible to increase the output of the laser unit.